1. Field of the Invention
The present invention relates to a method of fabricating transparent conductive ITO films and, more particularly, to a method of fabricating transparent conductive ITO films used for electrodes of a liquid-crystal display or a solar cell by sputtering.
2. Description of the Related Art
As a transparent conductive film, a transparent conductive ITO film having In (indium) and O (oxygen) as basic constituent elements and having Sn (tin) added thereto as a donor has hitherto been known. This transparent conductive ITO film is fabricated by a chemical film formation process utilizing a chemical reaction, such as a spray process, a CVD process, a wet dipping process, or a physical film formation process utilizing a physical phenomenon in a vacuum such as a vacuum deposition process, or a sputtering process.
Of the above-described thin-film fabricating methods, the sputtering process is superior to the other film formation methods in that a transparent conductive ITO film having a relatively low resistivity can be obtained, and a transparent conductive ITO film having a uniform thickness can be formed on a relatively large substrate such as a glass plate.
For the sputtering process, a direct current (DC) discharge sputtering method and a radio frequency (RF) discharge sputtering method are available. At present, the direct current (DC) discharge method (called a DC sputtering process) is conventionally used among the sputtering processes because of its low cost, stable discharges, and easy control. For the sputtering process, a magnetron sputtering method is also available in which a plasma is converged on a surface of a target with a closed magnetic field produced by one or more magnets disposed behind the target. The magnetron sputtering method is also conventionally used among the sputtering processes because of its high-film deposition rate and therefore the magnetron sputtering method is available for mass production of integrated circuits. Based on the above, as a method of fabricating transparent conductive ITO films in mass production, a DC magnetron sputtering process in which a direct current discharge method and a magnetron method are combined is generally used at present. The DC magnetron sputtering process has recently been developed into a movable magnet mode in which the magnets disposed behind the target are reciprocated (oscillated) or eccentrically rotated to sputter the entire surface of the target.
In the sputtering process, in general, as main factors which exert an influence upon the resistivity of a transparent conductive ITO film, substrate temperature and an oxygen partial pressure are known. When the substrate temperature is increased, the resistivity of the film is decreased. When the oxygen partial pressure is decreased, a number of oxygen vacancies are produced within the film fabricated in that atmosphere. A film having a number of oxygen vacancies has a high carrier density, but, in contrast, the mobility of carriers is low. On the other hand, in the film fabricated in an atmosphere having a high oxygen partial pressure, the number of oxygen vacancies is small, and therefore the carrier density is low, but the mobility of carriers is high. Since the resistivity is proportional to the inverse of the product of the carrier density and the mobility of the carriers, there is a most appropriate oxygen partial pressure at which the resistivity becomes a minimum due to balance between the carrier density and the carrier mobility. Therefore, in the conventional sputtering process, the resistivity of a transparent conductive ITO film is decreased by adjusting each of the substrate temperature and the oxygen partial pressure as parameters.
In the fabrication of transparent conductive ITO films by using the above-described DC magnetron sputtering process, the deposition rate is usually approximately 100 nm/min. This deposition rate is affected by various factors, such as the spacing between the target and the substrate, and the density of the target (or the filling density). However, the deposition rate is mainly determined by the power applied to the target. In the case of an ordinary stationary magnet mode, the power to be applied to the target is about 1 to 2 W/cm.sup.2, determined by dividing the power to be applied to the target by the eroded area of the target. In the case of a movable magnet mode (oscillating motion or eccentric rotation), the power to be applied to the target is about 1 to 2 W/cm.sup.2, as determined represented by dividing the power to be applied to the target by the eroded area of the target formed when the magnet used in the movable magnet mode remains stationary.
The density of the target (the filling density) means the ratio of the actual target density to the theoretical density calculated from the crystalline structure of an oxide of indium (In.sub.2 O.sub.3). The value represented by dividing the power to be applied to the target by the eroded area of the target is called "power density". In the case of a movable magnet mode, the value represented by dividing the power to be applied to the target by the eroded area of the target formed when the movable magnet remains stationary is called "power density".
When transparent conductive ITO films are continuously fabricated by DC magnetron sputtering at a power density of about 1 to 2 W/cm.sup.2 described above, the resistivity value of the transparent conductive ITO film is increased gradually with an increase in the number of times of the fabrication of the films (or the number of films deposited). Therefore, the resistivities of all the obtained transparent conductive ITO films are not the same. Consequently, there arises a problem that the resistivities of the transparent conductive ITO films, which are continuously fabricated by a DC magnetron sputtering process, increase as sputtering proceeds.
FIG. 3 shows a change in the resistivity of a film with respect to the cumulative power applied to a target when transparent conductive ITO films are continuously fabricated on a glass substrate by a DC magnetron sputtering process with the power density being set at 1.0 and 2.0 W/cm.sup.2. Here, the cumulative power means the number of times of the fabrication or the number of films deposited in the continuous fabrication of transparent conductive ITO films. The continuous fabrication of transparent conductive ITO films is performed in a single substrate processing mode in which a film is deposited on each substrate by one sputtering process by using a sintered target (density: 95%) having 10 wt. % of SnO.sub.2 added to In.sub.2 O.sub.3 at a substrate temperature set at 200.degree. C. and at a pressure of sputter gas of 0.4 Pa. The sputter gas is a mixture of Ar and O.sub.2 gases, and the concentration of the O.sub.2 gas in the sputter gas is adjusted in such a manner that the resistivity becomes a minimum every 3 kWh of the cumulative power. It can be seen from FIG. 3 that the resistivity at the power density of 2.0 W/cm.sup.2 is smaller in the rate of the increase than at 1.0 W/cm.sup.2. However, the resistivity increases as the cumulative power increases for both of the power densities.
As shown in the above-described example, when the films are continuously fabricated by a magnetron sputtering process at a power density of about 1 to 2 W/cm.sup.2, the resistivities of all the transparent conductive ITO films fabricated are not in a desirable range. Conventionally, by mechanically shaving off the surface of the target before reaching the end of the target life, resistivity is kept within a predetermined range required to guarantee the performance of a device, such as a liquid-crystal display or a solar cell. Such redundant operations during the continuous fabrication cause problems, for example, a low productivity, or a high manufacturing cost.
FIG. 3 shows that the resistivity of the film increases as the cumulative power increases when transparent conductive ITO films are continuously fabricated. Such an increase of resistivity of the film with respect to the cumulative power, strictly speaking, occurs during film deposition on one substrate. That is, the resistivity of the film on one substrate increases along the thickness of the film. Therefore, in the case of depositing a film, to some extent a thick film, there arises a problem that the resistivity of the film, as a whole, increases as the film thickness increases.
It is an object of the present invention to solve the above-described problems. It is another object of the present invention to provide a method of fabricating transparent conductive ITO films having a low resistivity within a predetermined range until the end of the target life while continuously fabricating transparent conductive ITO films by magnetron sputtering. It is a further object of the present invention to provide a method of fabricating transparent conductive ITO films having a predetermined low resistivity even if the film thickness is great, while fabricating a transparent conductive ITO film on a single substrate.
It is a still further object of the present invention to provide a method of fabricating transparent conductive ITO films having a low resistivity from a view point of controlling the power density to be applied to the target. It is a still further object of the present invention to provide a method of fabricating transparent conductive ITO films capable of enhancing the efficiency of the target utilization and, as a result, enhancing the productivity of transparent conductive ITO films.
In accordance with a first aspect of the present invention, in a method of fabricating transparent conductive ITO films by a magnetron sputtering process, a sintered mixture of oxides of In and Sn being used as a target, the transparent conductive ITO film having In and O as basic constituent elements and added Sn as a donor, the method comprising a first step of depositing a transparent conductive ITO film on a substrate in an atmosphere produced by an inert gas and an oxygen gas, and a second step of removing, after the first step is stopped, a superficial oxygen-deficient layer of the target formed during the first step by means of electric discharge at a power density at which the rate at which the target is eroded is faster than the formation rate of the superficial oxygen-deficient layer, the first and second steps being alternately repeated.
In accordance with a second aspect of the present invention, the above-described power density is determined according to the density of the target in the first aspect of the present invention.
In accordance with a third aspect of the present invention, when the density of the target is 95% the power density is 2.5 W/cm.sup.2 or more in the second aspect of the present invention.
In accordance with a fourth aspect of the present invention, when the density of the target is 70%, the power density is 4 W/cm.sup.2 or more in the second aspect of the present invention.
In accordance with a fifth aspect of the present invention, the target is cooled by cooling means in the first aspect of the present invention.
In accordance with a sixth aspect of the present invention, in the first aspect of the present invention, when the resistivity of the transparent conductive ITO film has reached the limit of the resistivity required to guarantee the performance of the device in which the transparent conductive ITO film is used, the first step shifts to the second step, the second step is terminated after the superficial oxygen-deficient layer of the target is removed, the first step is started again, thereafter the first and second steps are alternately repeated in the same procedure.
In accordance with a seventh aspect of the present invention, a transparent conductive ITO film is continuously formed on each of a plurality of substrates in the first step, and thereafter a second step is performed in each of the above-described aspects of the present invention.
In accordance with an eighth aspect of the present invention, the first and second steps are alternately repeated on one substrate in each of the above-described aspects of the present invention.
In accordance with a ninth aspect of the present invention, a mixture of oxides of In and Sn is used as a target, a transparent conductive ITO film having In and O as basic constituent elements and added. Sn as a donor deposited in an atmosphere produced by an inert gas and an oxygen gas by a magnetron sputtering process, film deposition takes place at a power density at which a rate at which the target is eroded faster than the rate of formation of the superficial oxygen-deficient layer.
In accordance with a tenth aspect of the present invention, the power density is determined according to the density of the target in the ninth aspect of the present invention.
In accordance with an eleventh aspect of the present invention, when the density of the target is about 95%, the power density is from 2.5 W/cm.sup.2 to less than 4 W/cm.sup.2 in the tenth aspect of the present invention.
In accordance with a twelfth aspect of the present invention, when the density of the target is about 70%, the power density is 4 W/cm.sup.2 or more in the tenth aspect of the present invention.
In accordance with a thirteenth aspect of the present invention, the target is cooled by cooling means in each of the above-described ninth to twelfth aspects of the present invention.
In the present invention, the rate relation condition that the rate at which the target is eroded is faster than the formation rate of a superficial oxygen-deficient layer is satisfied by controlling the power density. The superficial oxygen-deficient layer formed on the surface of the target is removed by the control of the power density according to the rate relation condition. The control of the power density makes it possible to prevent the oxygen concentration of the target superficial layer from decreasing as the fabrication of the transparent conductive ITO film proceeds. Therefore, the control of the power density makes it possible to prevent an increase of the resistivity of the film caused by the formation of the superficial oxygen-deficient layer of the target. Consequently, the control of the power density makes it possible to maintain the resistivity of the film fabricated by DC magnetron sputtering within a relatively low predetermined range even if (a) transparent conductive ITO films are continuously fabricated on each of a plurality of substrates, or (b) a transparent conductive ITO film having a fairly great thickness is fabricated on a single substrate. Furthermore, since the film may be fabricated efficiently up to the end of the target life by removing the superficial layer of the target, the control of the power density enhances the efficiency of the target utilization. In addition, since the superficial layer of the target is removed, redundant operations, such mechanically shaving off the surface of the target, or replacing the target, are not necessary. Thus, the control of the power density leads to improved productivity of transparent conductive ITO films.
With the power density being set at a value which satisfies the above-described rate relation condition, transparent conductive ITO films can be continuously fabricated on each of a plurality of substrates by DC magnetron sputtering. However, in the continuous fabrication of transparent conductive ITO films, it is possible to include a second step in which magnetron sputtering is performed at a power density which satisfies the above-described rate relation condition when the resistivity of the transparent conductive ITO film has reached an upper limit of resistivity required to guarantee the performance of a device in which the transparent conductive ITO film is used during the first step in which film deposition is performed at a power density which is most appropriate for film deposition. Furthermore, it is possible to alternately perform the first and second steps in the continuous fabrication of transparent conductive ITO films. In such continuous fabrication also, the superficial oxygen-deficient layer of the target formed during the continuous fabrication is removed in the second step. The removal of the superficial oxygen-deficient layer in the second step makes it possible to restore the surface of the target to the same state as its initial use in the next first step. Therefore, in the second step, the resistivity of the film which has increased gradually during the continuous fabrication is returned to the value obtainable in the initial use of the target.
The above and further objects, aspects and novel features of the invention will more fully appear from the following detailed description when read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended to limit the invention.